Storage system



y 1962 J. F. NICHOLON 3,04,43l

STORAGE SYSTEM Filed Dec. 22, 1958 2 Sheets-Sheet 1 Fig.2.

WITNESSES INVENTOR 1%; James F. Nicholson July 24, 1962 J. F. NICHOLSON STORAGE SYSTEM 2 Sheets-Sheet 2 Filed Dec. 22, 1958 Fig.3.

Time in Seconds iii 82 Fig.4.

ififiik 354 139 H E Target Volts 5mm 9. 222 m 3 6 E B 2.36m

Time in Minutes Patented July 24, 1962 3,046,431 STORAGE SYSTEM James F. Nicholson, Elmira, N.Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 22, 1958, Ser. No. 782,035

Claims. (Cl. 313-65) member upon which is established a pattern of charges 1 representative of the stored image. The charge pattern is normally established by ,use of an electron beam depending on secondary emission or electron bombardment induced conductivity of the target. The charge pattern established or written on the target may then be read by an electron beam.

In a storage tube of the type described above it is desirable in many applications to perform the operation of reading without removing the charge image or pattern and thus read as many times as required. This is sometimes referred to as multicopy. It is also desirable that such a tube provide half-tones, that is, signals lying between the maximum and minimum signals applied to the tube in all copies.

At the present time there are at least two types of storage tubes, the metrechon and the transmission type storage tube, which can be utilized for producing copies of previously stored electrical information on a multicopy basis. In both these devices the information is stored electrically on the surface of a storage member of an insulating material. The multicopy feature is achieved in the metrechon by permitting electrically stored information to modulate a scanning beam. In the case of the transmission type tube, a flood beam is required which is modulated in accordance with a charge pattern on an apertured storage member. In both of these devices, it is necessary to bias the storage surface negatively to prevent landing of the electron reading beam. The metrechon requires very intricate mechanical structure which is expensive to manufacture and which is critical in operation. The transmission type storage tube has achieved wider use in the industry but this tube is limited in resolution as well as the ability to provide the necessary halftones. An additional problem of the transmission type storage tube is the difficulty of achieving optical coupling to the storage surface.

It is therefore an object of my invention to provide an improved storage device in which it is possible to derive many copies of the stored pattern without removing the stored pattern.

It is another object to provide an improved storage tube in which it is possible to resolve half-tone signals.

It is another object to provide an improved storage tube capable of receiving, storing and providing multicopy output signals without loss of half-tone values within the copies.

It is another object to provide a charge storage tube which will store a charge pattern corresponding to a light pattern directed on said storage member for a given time and then retain the stored charge pattern after removal of the light While at the same time allowing the information to be read out on a multicopy basis.

It is another object to provide a charge storage tube providing up to 100,000 copies of a stored image without substantial reduction in the half-tone values in the image.

My invention is primarily directed to a charge storage tube having a semi-conductor target storage layer. The material in the storage target has its conducting properties modified when subjected to irradiation or corpuscular bombardment.

My device provides a storage tube which exhibits certain electrical properties particularly as applied to its memory of its own electrical and optical history. More specifically, my device provides a target which under the stimulus of electron bombardment can remember and retain previously applied potential on the target. The improved target or storage member has the ability to hold previously written information by either an electron excitation or a radiation excitation through use of a low velocity electron beam scanning the target or through use of a flood beam and at the same time provide read out of the stored information on a multicopy basis.

These and other objects of my invention will be apparent from the following description taken in accordance with the accompanying drawing throughout which like reference characters indicate like parts, and in which:

FIG. 1 is a charge storage tube partially in section in accordance with the teachings of my invention;

FIG. 2 is an enlarged sectional View of a target used in the tube of FIG. 1;

FIG. 3 is a graphical showing of the difference in dark current between the device shown in FIGS. 1 and 2 and a vidicon type pick-up tube;

FIG. 4 is a graphical showing of the signal holding and dark currents within the device shown in FIGS. 1 and 2;

FIG. 5 is a graphical showing of the decay properties of a vidicon type pick-up tube; and

FIG. 6 is a graphical showing of the decay properties of a device according to the teaching of my invention.

Referring in detail to FIG. 1, there is illustrated a light sensitive storage tube. The envelope, electron gun, scanning and deflection systems are similar to the conventional vidicon. The vidicon is a well-known television pick-up tube for producing a video signal for transmission. The tube shown in FIG. 1 comprises an envelope 10 of a suitable material such as glass. One end of the envelope is closed by an end wall or face portion 12 through which electromagnetic Waves such as light from the scene viewed enters and is directed onto an input screen 14. The end Wall 12 is of a suitable material transmissive to the radiation to be detected. Glass may be used for visible light. The interior surface of this viewing window 12 is provided with an electrically conductive coating 16 transmissive to the radiations from the scene. A suitable material for the coating 16 is stannic oxide. The input screen or storage member 14 consists of a first layer 13 deposited on the electrically conductive coating 16 and a second layer 15 deposited on the layer 13. The coating 16 is connected to the ex terior of the envelope by lead 60. The lead is connected to one terminal of resistor 62 with the other terminal of the resistor 62 connected to a voltage source 64. This supplies a potential to the coating 16 of about 1-0 to 50 volts. The other end of the tube envelope 10 provides the base of the tube through which various leads (not shown) enter the tube for applying suitable potentials to the electrodes therein. The tube is provided with the necessary and well-known components needed to produce a beam of electrons and control this beam so as to scan the beam across the input screen 14 in a point by point manner. An electron gun 17 is provided within the tube adjacent the base to generate and 0 form an electron beam. The electron gun 17 may be of suitable design and includes a cathode 20, a control grid 28, a screen grid 30 and an accelerating electrode 32. The cathode is comprised of a tubular sleeve 22 closed at one end facing the screen 14. The closed end of the sleeve 22 is provided with a suitable thermionic emissive material coating 24 to provide electron emission. A heater 26 is provided within the sleeve to heat the coating 24. The electrons emitted from the cathode 2% are formed into an electron beam by the control grid 28 and screen grid 30. The cathode 20 is connected to a suitable voltage source 34 which may be at ground. A source 36 of video signals may be connected to the cathode 20 by a switch 38 in some applications of my invention.

The control grid 28 is supplied with a suitable negative potential of about 45 to 95 volts from a source 40. It may be desirable in some application to modulate the control grid 28 by connecting the video source 36 to the control grid 28. The screen or accelerator grid is supplied with a positive potential of about 300 volts from a source 42. The beam focus electrode 32 is supplied with a positive potential of about 250 volts from a source 44.

The electrons from the gun 17 are magnetically focused to a small area beam at the target 14 by a magnetic field derived from focus coil 50. Alignment coils 52 may also be provided to correct for misalignment of the gun 17. A deflection yoke 54 is provided to provide means of scanning the electron beam over the surface of the target 14. During operation, suitable voltages are applied to the deflection system 54 to provide the necessary scan.

A fine mesh screen 56 is mounted adjacent to the target 14, is electrically connected to the electrode 32, and is operated at the same potential.

In the conventional vidicon, the electron beam may scan the surface of the input screen either at high or low velocity. In the low velocity type operation, the electrons approach at a velocity below the first cross-over potential of the film of material on the target. The electrons are deposited on the target and will tend to charge the surface in a negative direction toward an equilibrium potential which approaches the cathode potential of the gun. If operated at the high velocity, the electrons strike the surface of the target between the first and second cross-over potential of the material and the targ t surface will tend to be charged in a positive direction by the electron beam. In the case of the low velocity scan, the conductive back plate is connected through a resistance to establish a potential of about 10 to 50 volts positive with respect to the equilibrium potential.

Due to the photoconductive properties of the input screen of the vidicon when light is focused upon the film, a current flow will take place through the film in the illuminated areas. This induced current will charge the illuminated areas toward the potential of the conductive coating or back plate. This flow of current discharges the scanned surface of the film from its equilibrium value toward the back plate potential. Those areas not illuminated by light will have little or no current flow depending upon the operation of the photoconductive material in the dark and thus will remain at the equilibrium potential established by the electron beam. As the electron beam scans over the input screen, those discharged areas illuminated by light will rapidly be driven or charged back to the equilibrium potential by the electron beam. Since the back plate is capacitively coupled to the screen surface of the target, the instantaneous charging of the target by the beam to equilibrium potential will provide potential changes across the resistance connected to the back plate. The changes in potentials across the output resistance are amplified in a well known manner to provide the output signal.

The vidicon type pick-up tube has been discussed in order to explain and appreciate the contribution of this invention. The vidicon uses a target of a layer of photoconductive material. In darkness, the resistivity of the photoconductive material is about 10 ohm-centimeters or more. The time constant of the target of a vidicon is materially greater than the time between successive scanning. In the case of television, this is of a second. When the photoconductive material is excited by a photon of light or other radiant energy, the resistance of the layer is lowered and allows current to flow thereby reducing the voltage drop across the area.

The scanning beam in the vidicon deposits 21 sutficient number of electrons on each area to replace the charge removed by the photoconduction. This action erases the information which has been written on the target by the action of the light.

In my device, I provide a system in which the information written on the target is not erased by the reading beam. I desire that the stored information be retained while at the same time provide many copies of the stored information. I have provided such a system by providing a target 14 of suitable material. The coating 13 is a homogeneous mixture of arsenic and selenium, for example, 300 milligrams of high purity selenium and milligrams of chemically pure arsenic. This mixture may be placed in a hard glass test tube, such as pyrex and the mixture heated slowly to about 500 C. in atmosphere thus causing the selenium to melt and take the arsenic into solution. It is then necessary to continue to heat slowly with agitation until a homogeneous molten mixture is obtained. It is necessary to agitate the mixture to prevent segregation of the selenium. This heating is continued for approximately 5 minutes and a homogeneous mixture is obtained in the melt without evaporation of the selenium. After a homogenous mixture has been obtained, the mixture is allowed to solidify and cool. The vessel may then be broken away and the material stored in a clean container. The specific example given above provides a 2 to 1 ratio by weight of selenium to arsenic and has been found to yield excellent results. It also has been found that other proportions are possible, such as one to one.

The resulting homogeneous mixture of arsenic and selenium may then be evaporated onto the conductive layer 16. This is accomplished by placing the target member in a closed container capable of being evacuated. A small quantity, about 60 milligrams of the homogeneous mixture, is suitable for depositing a layer on an one-inch diameter target. The amount of material utilized will depend on the area and the thickness of the layer desired. The mixture is placed in a boat of suitable material, such as a Nichrome, inserted into the container and positioned at a distance approximately 4 inches from the target. The system is then exhausted to a pressure less than about .5 micron. The boat is heated to approximately 400 C. The temperature is dependent on the desired speed of evaporation. The heating is continued for approximately 3 to 4 minutes until a target thickness of about 5 microns is obtained. The color of the target will appear as a deep amber or slightly red color. The evaporator utilized consists of a circular disc which refleets light from a tungsten source of 11.30 watts and 2.7 amperes positioned at a distance of 10 inches from the target. The color of the light reflected from the evaporator after transmission through the target is observed during the target evaporation process. When a deep amber or slightly red color is obtained, the heat is turned off and the first layer is completed.

The system is then opened to the atmosphere and a boat is inserted containing about 60 milligrams of antimony trisulfide. This boat is also provided with a 400 line woven steel mesh over the boat and material to prevent splash. The antimony trisulfide is then heated to approximately 450 C. and the material is deposited onto the arsenic selenium layer until a deeper amber color is obtained. This second layer of antimony trisulfide is of about the same thickness as the arsenic and selenium layer and the proper thickness is determined by the light reflection as in the case of the first layer. This second evaporation step should be accomplished as soon as possible after'the container is opened so that the first layer is not exposed to the atmosphere for more than two minutes. The target, consisting of the arsenic selenium layer and the antimony trisulfidelayer, is then complete and is ready to be sealed into the envelope. Further preparation of the tube should not cause the coating temperature to exceed a temperature of =l C.

The antimony trisulfide may be prepared by weighing high level source of illumination on the target.

out, for example, 18.4 grams of antimony to 6.7 grams of sulfur. The two materials are mixed together, placed in a test tube, and flushed with a slow stream of argon. The mixture is then slowly heated in a furnace at a temperature of 1000? C. to provide that the sulfur does not boil out of the top of the test tube. The mixture is then allowed to cool with the argon gas still flowing. The ingot is then removed, placed into another test tube, again flushed with argon, and the temperature raised to 600 C. for a period of one hour. The ingot is then allowed to cool and the upper half of the ingot will provide suitable antimony trisulfide for the layer.

It is found that when this target member is positioned within the tube, an unexpected and significant change is obtained over the conventional vidicon. If a light signal is impressed on the target and the lens then capped to prevent any further illumination of target with suitable scansion as used in the conventional vidicon, it would normally be expected that an image on the monitor would extinguish'pr-actically instantaneously. It has been found that the image does not extinguish and the image can be retained anywhere from minutes to a hour on the conventional thirty frames per second television type of scanning operation. It is found that the low velocity reading electron beam does not erase the charge as in the conventional vidicon.

FIG. 5 illustrates the typical decay of the charge on the target of a conventional vidicon with conventional television scan rate.

FIG. 6 illustrates the decay of the charge on the target of my device with conventional television scan rate. It is found that images viewed on the monitor 1 to 10 minutes after the camera lens has been capped have very little degradation. A standard vidicon will not hold an image more than about one or two frames which is equivalent to less than one second. My tests have indicated that the ability to hold the stored signal is due to the action of the scanning beam. If the scanning beam is biased off or if the target voltage is dropped to zero for more than a few seconds, the image 'extinguishes. Otherwise the image can be held for an indefinite time.

In FIG. 3, the curves shown illustrate the dark current under different conditions as a function of time starting after complete erase and the tube capped to prevent illumination of the target. Curve 70 illustrates 'the dark current in my device with a target voltage of 4 volts and the cathode current adjusted to 100 micro amperes. Curve 72 illustrates the dark current in my device with the cathode current adjusted to 200 microamperes. Curve 74 illustrates the dark current of a from curve 84 to 82 and not to curve 80 as would be expected in a conventional vidicon. FIG. 4 clearly shows the charge retention proper-ties of my device.

Although I have described my invention in only one specific embodiment, the target member has a wider variety of tube applications in the image and storage field. The target provides a member in which the charge pattern may be written on the target by radiation and the pattern read-out by electrical means. It is, of course, possible to write the information electrically on the target and read-out electrically. The device can also be utilized for single copy writing and multicopy reading. It may also be used for multicopy writing and with multicopy read-out. It is also possible to use an integration mode in which weak signals are strengthened and built up over a period of many frames of writing. The surface has an advantage in this later application because reading out the weak signals does not discharge the areas.

One obvious application of the specific embodiment described herein is to write an optical image on the surface in a relatively short time and view the image leisurely for extended periods of time. The device also provides a tube in which it is possible to apply selective erasure techniques, if necessary, by utilizing a light scanning beam. It is also possible by adjusting the target voltage to control the persistence. This will permit rapid or slow viewing. It is also possible that the device may be made sensitive to other types of radiation. This may be accomplished by a conversion layer. For example, an X-ray sensitive phosphor may be utilized in the case of X-ray input. This phosphor is selected to emit radiation to which the storage layer is sensitive. It is also possible to achieve multicopy effect by disposing a layer of the material over present photoconductors.

While I have shown my invention in only one form it will be obvious to those skilled in the art that it is not so limited or intended for various other changes Without departing from the spirit and scope thereof.

I claim as my invention:

1. A storage member adapted to receive and to retain an electrostatic charge for a substantial length of time comprising a support member having an electrically conductive coating on one surface of said support member and a layer of a mixture consisting essentially of arsenic and selenium deposited on the exposed surface of said conductive layer and a layer of antimony trisulfide on the exposed surface of said mixture layer.

2. A storage member adapted to receive and to retain an electrostatic charge for a substantial length of time comprising a support member having an electrically conductive coating on one surface of said support member and a layer of a mixture consisting essentially of arsenic and selenium deposited on the exposed surface of said conductive layer of about 5 microns in thickness and a second layer of a photoconductive material deposited on said mixture layer.

3. The method of producing and providing multicopy readout of a spaced distributed stored image which comprises providing an electrical conductive layer, a homogeneous surface of semiconductive material thereon exhibiting the property of storing a charge image under electrical signal from said electrical conductive layer in response to impingement of the electrons on said homogeneous surface of said semiconductive material layer.

4. A storage tube comprising a target member, said target member including an electrically conductive electrode member, electromagnetic wave sensitive means provided on said conductive member for storing an information pattern including half tones in response to electromagnetic waves and discharging said pattern in a predetermined time in the absence of electron bombardment, said means also for retaining said stored pattern for a greater length of time than said predetermined time in response to a constant current of electrons, means for directing electromagnetic waves onto said target to establish a storage conductivity pattern in said target and means for directing an electron beam onto said target to retain said stored pattern for a greater length of time than said predetermined time.

5. A storage tube comprising a target member, said target member including an electrically conductive electrode, means comprised of at least one substantially continuous layer provided on said conductive electrode for storing a pattern including half tones in response to excitation and discharging said pattern in a predetermined time in the absence of electron bombardment, said means also for retaining said stored pattern for a greater length of time than said predetermined time in response to a constant current of electrons, means for directing excitation onto said target to establish a stored pattern on said target subject to discharge in a predetermined time in the absence of electron bombardment and means for scanning an electron beam over said target at a predetermined rate to retain said stored pattern for a greater length of time than said predetermined time.

6. A storage tube comprising a target member, said target member including an electrically conductive electrode member, electromagnetic wave sensitive means provided on said conductive electrode for storage of a pattern including half tones in response to electromagnetic Waves and discharge of said pattern in a predetermined time in the absence of electron bombardment, said means also for retaining said stored pattern for a greater length of time than said predetermined time in response to a constant electron excitation, means for directing electromagnetic waves onto said target to establish a stored pattern on said target and means for exciting said target with a constant current source of electrons at an energy below the first cross over potential of said target at a predetermined pulse rate to retain said stored pattern beyond said predetermined time.

7. A storage tube comprising a target member, said target member including an electrically conductive electrode member, light sensitive means provided on the surface of said conductive electrode for storage of a pattern including half tones in respone to light excitation and discharge of said pattern in a predetermined time in the absence of electron bombardment, said means also for retaining said stored pattern for a greater length of time than said predetermined time in response to constant electron excitation, means for directing a light image onto said target to establish a stored pattern of said target and means for directing an electron beam onto said target for retaining said stored pattern for a greater length of time than said predetermined time.

8. A storage tube comprising a target member, said target member including an electrically conductive electrode member, means provided on said conductive electrode to provide a homogeneous exposed surface for storage of a pattern including half tones in response to electron excitation and discharge of said pattern in a predetermined time in the absence of electron bombardment, said means also for retaining said stored pattern for a greater length of time than said predetermined time in response to a constant electron excitation, means for directing an 8. electron beam onto said homogeneous surface of said target to establish a stored pattern. on said target and means for directing a constant current electron beam onto said homogeneous surface to retain said stored pattern for a greater length of time than said predetermined time.

9. A storage tube adapted to receive and retain an electrostatic charge for a substantial interval of time comprising a support member having an electrically conductive layer provided on one surface thereof, means provided on the surface of the electrical conductive layer to provide a homogeneous surface for storing a charge image in response to excitation and discharging said image in a predetermined time in the absence of electron bombardment, said means also for retaining said charge image for a greater length of time than said predetermined time in response to bombardment of said homogeneous surface by a low energy scanning beam, said means including a first and a second layer of different photoconductive materials, means for directing a low energy scanning beam of electrons onto said homogeneous surface to generate a signal at the electrical conductive layer corresponding to said charge image and also introducing conduction into said target means such that the charge image is not substantially reduced by the signal generating action of said electron beam.

10. In a storage tube, a storage target, said target including a continuous layer of electrically conductive material and means provided on the surface of said electrically conductive material to provide a homogeneous surface for storing a charge image in response to radiation and discharging said image in a predetermined time in the absence of electron bombardment, said means also for retaining said image for a greater length of time than said predetermined time in response to bombardment of said homogeneous surface by low energy electrons, said means of a high resistant material for establishing a charge image thereon in response to radiation, means for directing a radiation image onto said target to establish a charge pattern on the surface of said target and means for retaining the charge pattern on said target including directing low energy electrons onto said homogeneous surface of said target such that the energy of the electrons striking said target is dependent on the charge pattern on said target to thereby retain the charge pattern including half tone qualities therein over a substantial length of time.

11. A storage display system comprising a storage target, said target comprising an electrically conductive electrode member, means provided substantially on the surface of said conductive electrode to provide a homogeneous surface for storing'a charge image in response to excitation for a predetermined time in the absence of electron bombardment, said means also for retaining said stored image for a greater length of time than said predetermined time by means of electron bombardment, means for directing excitation energy onto said target to establish a charge pattern on the target, means for generating electrons of substantially equal low velocity and directing said electrons onto said homogeneous surface of said target to retain said charge pattern, said electrons striking said homogeneous surface with energy dependent upon the charge pattern on said storage member and thereby retaining said stored image for a substantial length of time, means for directing a reading beam onto said homogeneous surface of said target to derive an electrical signal from said conductive layer representative of the charge pattern on said target and means for erasing said charge pattern including a source of light and means for directing the light from said source onto said target at given intervals to erase said charge pattern from said storage target.

12. A storage tube comprising a target member, said target member including an electrically conductive electrode, said electrode having means provided thereon for permitting the conductivity of said target to be set at a predetermined value in response to and substantially proportional to light excitation directed thereon and mainconductive material, means provided on said electrically conductive material to provide a homogeneous surface for permitting the conductivity to be set at a predetermined value in response to and substantially proportional to excitation directed onto said target and maintaining this conductivity substantially unchanged by a constant current of low energy electrons passing through said material, means for directing excitation onto said target to form a stored image thereon, means for generating a beam of low energy electrons of constant current, means for scanning said homogeneous surface of said target with said beam of electrons to generate a plurality of sequential signals representative of the signals applied to said target electrode, said electron beam simultaneously maintaining the conductivity substantially unchanged Within said target so that a plurality of copies may be obtained from said pick-up tube Without modifying the stored pattern in said target.

14. A storage tube comprising a target member, said target member including an electrically conductive electrode member and a layer of a mixture of arsenic and selenium for storing an information pattern including half tone in response to excitation and discharge said pattern in a predetermined time in the absence of electron bombardment, said layer also for retaining said stored pattern for a greater length of time than said predetermined time in response to a constant current of electrons, means for directing excitation onto said target to establish a storage conductivity pattern in said target and means for directing an electron beam onto said target to retain said stored pattern for a greater length of time than said predetermined time.

15. A storage tube comprising a target member, said target member including an electrically conductive electrode member and a layer of a mixture of arsenic and selenium of a thickness of about 5 microns provided on saidconductive member for storing an information pattern including half tones in response to excitation and discharging said pattern at a predetermined time in the absence of electron bombardment, said layer also for retaining said stored pattern for a greater length of time than said predetermined time in response to a constant current of electrons, means for directing excitation onto said target to establish a storage conductivity pattern in said target and means for directing an electron beam onto said target to retain said stored pattern for a greater length of time than said predetermined time.

References Cited in the file of this patent UNITED STATES PATENTS 2,687,484 Weimer Aug. 24, 1954 2,699,512 Sheldon Jan. 11, 1955 2,739,258 Sheldon Mar. 20, 1956 2,787,724- Webley Apr. 2, 1957 2,813,998 Haefi Nov. 19, 1957 2,839,679 Harris June 17, 1958 2,908,836 Henderson Oct. 11, 1959 2,910,602 Lubszynski et al Oct. 27, 1959 

1. A STORAGE MEMBER ADAPTED TO RECEIVE AND TO RETAIN AN ELECTROSTATIC CHARGE FOR A SUBSTANTIAL LENGTH OF TIME COMPRISING A SUPPORT MEMBER HAVING AN ELECTRICALLY CONDUCTIVE COATING ON ONE SURFACE OF SAID SUPPORT MEMBER AND A LAYER OF A MIXTURE CONSISTING ESSENTIALLY OF ARSENIC AND SELENIUM DEPOSITED ON THE EXPOSED SURFACE OF SAID CONDUCTIVE LAYER AND A LAYER OF ANTIMONY TRISULFIDE ON THE EXPOSED SURFACE OF SAID MIXTURE LAYER. 